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vol. 50 num. 1 lang. eshttp://www.scielo.cl/img/en/fbpelogp.gifhttp://www.scielo.cl
Metal complexes of a new ligand derived from 2,3-quinoxalinedithiol and 2,6-bis(bromomethyl)pyridine]]>http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0717-97072005000100001&lng=es&nrm=iso&tlng=es
The synthesis of a new ligand containing quinoxaline and pyridine subunits is described. The reaction of 2,3-quinoxalinedithiol with 2,6-bis(bromomethyl)pyridine leads to the isolation of bis(2-thio-3-mercaptoquinoxalino)-2,6-dimethylpyridine (L), which is a macrocyclic ligand precursor. The reaction of transition metal ions with L gives [M(L)X]X complexes (where M = Mn(II), Co(II), Cu(II), Ag(I), Zn(II), or Cd(II), and X = Cl or Br). The compounds were characterized by physical and spectroscopic measurements which indicated that the ligand is probably acting as a pentadentate NS4 chelating agentElectrocatalysis of NO2- on poly-M-tetraaminophenylporphyrines (M = Ni, Cu) modified electrodes assisted by visible light]]>http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0717-97072005000100002&lng=es&nrm=iso&tlng=es
Porphyrins and phthalocyanines of transition metals are good catalysts for many redox reactions [<A HREF="#1">1</A>-<A HREF="#7">7</A>]. In order to increase their catalytic activity it is often necessary to modify the macrocycle with donor or acceptor substituents making the species more reductive or oxidant depending on the analyte. However, this process is a one-way modification, because the molecule enhances or diminishes the electronic density of the metal by the substituent effectOxime-Derivatives of Dihydroeuparin]]>http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0717-97072005000100003&lng=es&nrm=iso&tlng=es
The synthesis of derivatives of dihydroeuparin, a secondary metabolite of the shrub Senecio graveolens, is described. The main modifications were the introduction of a hydrocarbon branch and the transformation of a ketone group into its oxime. Structure identification was carried out by IR, ¹H-NMR and 13C-NMR spectroscopy. These compounds may have application in the process of copper extractionSTUDIES ON CHILEAN FUNGI III. FREE AND BOUND STEROLS FROM MYCENA CHLORINELLA* (BASIDIOMYCETES)]]>http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0717-97072005000100004&lng=es&nrm=iso&tlng=es
A mixture of oleic, linoleic and a-linolenic esters (1b-d, 2b-d) of ergosterol (1a) and episterol (2a) was isolated from the fruiting bodies of Mycena chlorinella. Ergosterol and its endoperoxide (3) were also found as free sterols. In addition, relatively large amounts of the nucleoside adenosine (4) were isolatedEffect of the nature of the support on the enantioselective hydrogenation of 1-phenyl-1,2-propanedione over supported iridium catalysts]]>http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0717-97072005000100005&lng=es&nrm=iso&tlng=es
The enantioselective hydrogenation of 1-phenyl-1,2-propanedione at 298 K and 40 bar over modified supported iridium catalysts has been studied. Cinchonidine has been used as chiral inducer. The catalysts were obtained by impregnation of Ir(acac)3 on three different supports: SiO2, TiO2 and MoO3, followed by calcination in air and reduction under hydrogen at 773 K. All the solids were characterized by nitrogen adsorption-desorption isotherms at 77 K, H2 chemisorption, XRD, TEM, TPR and XPS. It was found that Ir/SiO2 and Ir/TiO2 catalysts reduced at high temperatures, 773 K, possess similar metal particle size, close to 2.0 nm, eventhough the H/Ir ratio obtained from H2 chemisorption showed larger differences, with the H/Ir ratio being lower for titania- and molybdenum-supported iridium catalysts. In these samples, migration of the partially reduced supports, TiOx and MoOx moieties, on the metal crystals induce the creation of Irdelta+ species. TPR and XPS results confirmed that the metal component was not completely reduced. The activity was influenced by the nature of the support, being more active those in the SMSI state such as Ir/TiO2 and Ir/MoO3 being more active. This has been attributed to the presence of electron deficient metal species, Irdelta+, which are responsible for the polarization of the carbonyl bond of the substrates, thus favoring the activity and enantioselectivity of the reaction. The effect of different solvents on the activity and the enantioselectivity of the reaction was also studied. The highest enantiomeric excess (ee) of (R)-1-phenyl-1-hydroxy-2-propanone (20%) was obtained with the Ir/TiO2 catalyst using acetic acid as solventPolymeric ligand-metal acetate interactions: Spectroscopic study and semi-empirical calculations]]>http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0717-97072005000100006&lng=es&nrm=iso&tlng=es
Semiempirical calculations on the dihydrated Zn (II) acetate and monohydrated Cu(II) acetate structures have been carried out. The optimized structures by the PM3 and AM1 methods were compared with crystallographic data in order to analyze the reliability of the data and its use in the calculations of the structures of the compounds obtained from poly (allylamine) with Zn(II) acetate and Cu(II) acetate. These compounds were characterized by means of elemental analysis, magnetic moments, FT-IR and electronic spectroscopy in the d-d region of the spectrum. The PM3 calculations of copper acetate monohydrate and the polychelate do not show differences. The Cu-Cu distance presents a difference of 0.04 A, which is in agreement with the high similarity, the electronic spectra and the magnetic measurementsSTUDY OF PEROVSKITES OBTAINED BY THE SOL-GEL METHOD]]>http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0717-97072005000100007&lng=es&nrm=iso&tlng=es
Perovskites are very thermally stable mixed oxides, frequently used as oxidation catalysts in spite of their relatively low surface area. In order to increase the area the sol-gel method has been used, with malic acid as complexing agent and pH adjustment of the starting solution, to prepare the LaCoO3 perovskite, used as a reference, and other perovskites in which the La cation has been partially replaced (20% molar) by Ca, Sr or Ba cations. Additionally, the cooling rate of the fresh catalysts, from the calcination temperature to room temperature, has been used as a parameter in the method of preparation. The catalysts were characterized by BET area, DRX, TG, DTA, and SEM-EDX. Catalytic activity was tested in a fixed bed flow reactor using the oxidation of CO with O2 reaction. The DRX studies confirmed the presence of a single phase with perovskite structure in the catalysts calcined at 550 C, and the additional studies confirmed that very uniform catalysts with great thermal stability were obtained. Substitution of La by group II cations and the method of preparation make it possible to obtain perovskites with large surface area and excellent catalytic activity for CO oxidation, reaching maximum conversion values, close to 100%, at temperatures of about 200 C. Changes in post-calcination cooling rate did not produce significant changes in the surface area or in the catalytic activity of the catalysts, except when Ba-substituted catalysts were usedPhysical Properties of the Phases A0.7Bi1.1P2Se 6 (A = Cu, Ag) and CuBi1-XSbXP2Se 6]]>http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0717-97072005000100008&lng=es&nrm=iso&tlng=es
The new selenophosphate phases A0.7Bi1.1P2Se 6 (A = Cu, Ag) and CuBi1-X SbX P2Se 6, stabilized by the chalcophosphate anions (P2Se6)4-, were prepared by molten polythiophosphate fluxes. The reaction products were characterized by atomic absorption (ICP) and Scanning Electron Microscopy (SEM-EDX), powder X-ray (XRD), Fourier transform infrared spectroscopy (FTIR) and a.c. and d.c. electrical conductivity measurements. The phases A0.7Bi1.1P2Se 6 and CuBi1-X SbX P2Se 6 are structurally related to AMP2Se6 and they possess two-dimensional (2D) structure. These phases are semiconductors, with values of electrical conductivity, sI, about 10-4 W-1cm-1 at room temperatureSynthesis of nanostructured materials by a new solid state pyrolysis organometallic polymer method]]>http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0717-97072005000100009&lng=es&nrm=iso&tlng=es
In this study, the preparation of nanostructured particles of metal/metal oxides by using a pyrolysis of organometallic polyphosphazenes having the metal anchored to the polymeric chain is described. The elaboration process is based on the thermal decomposition of organometallic derivatives of polyphosphazenes in air at temperature of 800 C°. As an example, the preparation of chromium, iron, ruthenium and manganese nanoclusters have been described, but the method is very general and a variety of meta/metal oxide materials can be obtained. Preliminary results on some special properties of the products are given and potential applications are discussedChiral Discrimination of L- and D-N-acyl-1-phenyl-d5-2-aminopropanes in a Cesium N-Dodecanoyl-L-Threoninate Cholesteric Nematic Lyomesophase]]>http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0717-97072005000100010&lng=es&nrm=iso&tlng=es
Molecular recognition based on chirality has fundamental importance in many biological processes. Deuterium quadrupole splittings from the aromatic ring and mesophase components of two series of optical isomers, L- and D-N-acyl-1-phenyl-d5-2-aminopropanes, dissolved in anionic nematic cholesteric lyotropic liquid crystals of cesium N-dodecanoyl-L-threoninate, were measured using²H-NMR. The length of the acyl chain was 1, 2, 3, 4, 5, 7 and 10 carbon atoms. The two order parameters that fully characterize the average alignment of the aromatic ring were calculated. Both the L- and D- isomers are strongly attached to the aggregate. L-C1, D-C1, L-C2 and D-C2 derivatives have the same order parameters which suggests that are located in the same region of the interface, possibly H-bonded to the interstitial water molecules with the NH and/or CO groups. Increasing the hydrophobic chain length by one carbon atom decreases the overall alignment of the ring and differences between L-C3 and D-C3 were observed. Molecules with longer acyl chain progressively increase their quadrupole splittings, suggesting an increase in alignment. Increasing differences in the order parameter of the symmetry axis of the aromatic rings in both isomers were observed from C3 to C5, and almost no differentiation is detected between L-C7 and D-C7. However, differentiation appears again for C10, and is attributable to interactions with a second chiral center of the head groupStyrene/1-alkene copolymerization by CpTiCl3-additive initiator systems]]>http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0717-97072005000100011&lng=es&nrm=iso&tlng=es
Initiator systems resulting from the combination of cyclopentadienyltitanium trichloride (CpTiCl3), ethylaluminium dichloride (EtAlCl2) with and without methylaluminoxane (MAO), were used to initiate styrene (S) homopolymerization and S/1-alkene copolymerization. The CpTiCl3-EtAlCl2 system turned out to be an effective initiator of styrene homopolymerization as well as of styrene/1-decene (S/1-C10H20) and styrene/1-hexadecene (S/1-C16H32) copolymerization. Both S/1-C10H20 and S/1-C16H32 copolymers obtained from various S/1-alkene molar ratios in the initial feed contained variable amounts of boiling-butanone-insoluble product which was a S/1-alkene copolymer according to NMR and DSC analyses. The copolymers obtained showed Tg values which decrease as the proportion of 1-alkene in the initial feed increasesSELECTIVE HYDRODECHLORINATION OF 1,2-DICHLOROETHANE OVER Pd-Sn/SiO2 CATALYSTS]]>http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0717-97072005000100012&lng=es&nrm=iso&tlng=es
Pd/SiO2 and Pd-Sn/SiO2 catalysts prepared by sol-gel processing have been characterised and tested in the selective hydrodechlorination of 1,2-dichloroethane. The obtained solids were characterised by N2 adsorption at 77 K, H2 chemisorption, temperature programmed reduction, and X-ray diffraction. It was found that the Pd surface atoms decreases upon the Sn addition, without critical changes on the specific area and reducibility. The turnover frequency increases continuously with the reaction temperature and also as tin loading increases. The selectivity to ethylene is strongly enhanced as temperature increases up to reach almost a constant value of 90 %, in the temperature range 500-600 K. It is proposed that Pd-SnOx are the active sites for the selective hydrodechlorination of 1,2-dichloroethaneINTERACTION ENERGIES IN NON WATSON-CRICK PAIRS: AN AB INITIO STUDY OF G·U AND U·U PAIRS]]>http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0717-97072005000100013&lng=es&nrm=iso&tlng=es
Ab initio calculations at the MP2/6-31G** level have been carried out on the non-Watson-Crick nucleic acids pairs G<FONT FACE=Symbol>&middot;</FONT>U and U<FONT FACE=Symbol>&middot;</FONT>U to obtain the interaction energies and to see whether the derived values are comparable or not with the canonical G-C, A-T and A-U pairs. Optimized geometries of the pairs show that the structural parameters of the isolated bases differ very little on pairing. The guanine -NH2 group does not participate in the hydrogen bonding formation and possesses a pyramidal structure; its intrinsic nonplanarity plays an important role in the out-of-plane intermolecular interactions. Thus, the G<FONT FACE=Symbol>&middot;</FONT>U pair projects three hydrogen bonding acceptor sites, namely, N7(G), O6(G) and O4(U) to the RNA major groove. The interaction energy (DHºint) calculated for the G<FONT FACE=Symbol>&middot;</FONT>U pair (-13.6 kcal/mol) is comparable to that determined for A-T (-13.0 kcal/mol), but considerable smaller than the experimental value reported for G-C (-21.0 kcal/mol). The U<FONT FACE=Symbol>&middot;</FONT>U pair follows the trend that pairing between pyrimidines bases should have lower interacting energies than purine-pyrimidine pairs